ORIGIN OF THE DRIFT 26o 



aptly termed "residual ablation deposits," or detritus slowly let down by 

 the removal of softer or more soluble material, since it is composed, not 

 only of material derived from the bedded volcanics, but also of intrusive 

 porphyries, evidence of which should be found today in the immediate 

 vicinity in the form of dikes or stocks, none of which, however, has been 

 discovered. Moreover, such an explanation would apply only to the 

 detritus on or near the divides and could not account for the material 

 occurring at lower levels near Montrose. Taking all these facts into 

 consideration, there seems to be sufficient ground for assuming that the 

 detritus can only be of glacial origin and is to be regarded as true drift. 

 From its position, that of West Baldy and Horsefly is most naturally to 

 be considered as lateral moraine, while the deposits occurring at lower 

 elevations near Montrose may represent remnants of the terminal moraine 

 or ground moraine. Detailed information in regard to those last occur- 

 rences is lacking, the mere fact of their existence and the general charac- 

 ter of their materials being known. 



The origin of the detritus between Cow creek and Cimarron ridge is not 

 altogether clear. The nature of the material indicates that it was de- 

 rived only from the early tuffs and agglomerates which constitute the 

 ridge, .and the abundance of recent landslides from the ridge presents the 

 possibility that this evidently older material may have had a similar 

 origin. There is one strong objection to accepting this as an explanation. 

 The material lying between the streams, and apparently only very slightly 

 affected by erosion since the time of its deposition, is usually less abun- 

 dant near the cliffs from which it was derived than it is farther away. 

 Its most characteristic occurrence also is in the form of mounds or heaps 

 of large and small blocks often separated from one another by several 

 hundred yards of bare ground covered only by the soil which results 

 from the weathering of the underlying Cretaceous shales. As has been 

 said, the composition of the material shows that it was derived from 

 Cimarron ridge, but its disposition could not be accounted for by the 

 simple agency of landslides. For the same reason the material could 

 hardly have been deposited by streams, and even torrents could not have 

 transported blocks from 10 to 30 feet in diameter three or four miles, 

 from their source. Water-laid gravels do occur, as is shown later, just 

 beyond the limits of this unstratilied detritus; they belong to the highest 

 series and rest upon the old land surface that has been frequently referred 

 to. It seems evident that these deposits can not be regarded as lateral 

 moraine of an early Cow Creek glacier corresponding in time to the 

 Horsefly deposits because none of the various types of rock known to 

 occur along the upper portions of Cow creek are found, the deposits con- 



